Vegan Guts? Meat's Glory?

TMAO, a substance found in abundance in the guts of meat eaters, has artery-clogging effects. But is meat the problem?

Self-proclaimed grill masters may want to rethink their cook out menus this summer, or at least offer their guests a veggie burger option. Red meat contains a compound, which has also been added to some energy drinks, that increases the risk of heart disease, according to a new study.

L-carnitine is a substance found in meat that helps the body turn fat into energy. It is made in the liver and kidneys and is required for the transport of fatty acids during the breakdown of lipids or fats.

A diet high in carnitine actually shifts our gut microbe composition to those that like carnitine, making meat eaters even more susceptible to forming TMAO and its artery-clogging effects.

When L-carnitine is digested in the gut, one of its breakdown products is trimethylamine-N-oxide (TMAO). Researchers have shown that TMAO promotes atherosclerosis, hardening or clogging of the arteries.

"The bacteria living in our digestive tracts are dictated by our long-term dietary patterns," Stanley Hazen, one of the authors, said in a statement. "A diet high in carnitine actually shifts our gut microbe composition to those that like carnitine, making meat eaters even more susceptible to forming TMAO and its artery-clogging effects.”

Hazen, who is section head of preventive cardiology and rehabilitation at the Cleveland Clinic, believes that vegans and vegetarians are less able to metabolize L-carnitine to TMAO, which may explain the cardiovascular health benefits of vegetable-based diets.

New Targets

“The authors describe a novel mechanism that appears to promote atherosclerosis which goes beyond the traditional risk factors we normally think about,” Vera Bittner, a professor of medicine at the University of Alabama told TheDoctor

in an email. The study opens up a whole new area of research and suggests treatment targets no one had previously considered. For example, one might reduce TMAO formation by changing a person's gut bacteria or targeting the compounds themselves.

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Vegans and vegetarians had levels of TMAO that were far lower than those of meat-eaters.

The investigators compared the L-carnitine and TMAO levels of those who ate both red meat and vegetables (omnivores) to those of vegans, and vegetarians. They also examined the clinical data on 2,595 patients undergoing elective cardiac evaluations. They found that increased L-carnitine levels in patients with high TMAO levels predicted an increased risk of cardiovascular disease and major cardiac events like heart attack, stroke, and death.

They also found that specific gut microbes were associated with both levels of TMAO in the blood and eating patterns. Vegans and vegetarians had levels of TMAO that were far lower than those of meat-eaters.

Meat seems to set up the body to overproduce TMAO. According to Hazen, when vegans and vegetarians ate a lot of L-carnitine, they still did not produce much TMAO. But omnivores consuming the same amount of L-carnitine produced high levels of the metabolite.

Is Carnitine Getting a Bad Rap?

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This is why not everyone agrees that L-carnitine is the source of the problem. Though TMAO may raise the risk of heart attack, L-carnitine may not be the culprit. Another study, an analysis of several controlled trials published online in Mayo Clinic Proceedings, found a significant reduction in heart problems among patients receiving L-carnitine following a heart attack, compared to those who didn't. So it may be something about meat's effects on gut microbes apart from L-carnitine that creates the overproduction of TMAO.

When vegans and vegetarians ate a lot of L-carnitine, they still did not produce much TMAO. But omnivores consuming the same amount of L-carnitine produced high levels of the metabolite.

Hazen sees risks, however. “[L-carnitine] is a form of amino acid. It helps cells transport fatty acids into mitochondria, the place where fuel is burned up to produce energy. This is why it is sometimes in [body building or fitness] supplements,” said Hazen. The idea that more L-carnitine can increase one’s energy level, and help burn fat, is not necessarily true in a normal person. People should not take it as a supplement just because they hope to improve their energy level, lose weight, or increase their muscle mass. Hazen recommends cutting back on red meat, the primary source of L-carnitine, in terms of portion size and the number of servings per week.

The findings seem to explain why people who eat red meat tend to have higher cholesterol and a greater risk of heart problems. But they do not mean that fat and cholesterol intake, and lifestyle issues such as smoking and physical activity level are not important, says Bittner. If you have been prescribed cholesterol medications by your doctor and you now decide to avoid red meat, you still need to take your cholesterol medications.

The study linking meat-eating to TMAO is published in Nature Medicine.